Electrolytic production of aluminum



United States Patent 3,103,472 I ELECTROLYTIC PRODUCTION OF Harvey L. Slatin, New York, N.Y., assignor to Trmax Associates, New York, N.Y., a partnership No Drawing. Filed Dec. 23, 1959, Ser. No. 861,431 6 Claims. (Cl. 204-67) This invention relates to the production of aluminum by fusion electrolysis.

This application is a continuation-in-part of my application concerned with the Electrolytic Production of Aluminum, Serial No. 613,718, filed October 3, 1956, now US. Patent No. 2,919,234, dated December 29,1959.

The instant invention provides a process for the production of aluminum by fusion electrolysis using an aluminum halide, such as aluminum chloride as a source of feed material,

The art contains records of attempts to produce aluminum from AlCl by electrolysis using an electrolyte composed of AlCl dissolved in alkali and/or alkaline earth halides. AlCl is a volatile solid :subliming at about 178 C. Although its vapor pressure may be suppressed in molten alkali chloride eutectic baths at [or about the melting point of aluminum (659.7 0.), AlCl is readily distilled from such baths. As a consequence these processes are operated at low temperatures or under a positive pressure. In the former instance, aluminum is deposited as a solid commingled with electrolyte that is diflicult to separate from the metal crystals. In the latter process, the equipment and labor are expensive and not proved practical.

Great interest in the use of A101 as a raw material is stimulated by the tact that present processes based on electrolysis of alumina consume from 0.6 to 1.0 pound of carbon per pound of aluminum. In addition, the high temperatures of operation cause severe corrosion, shorten equipment life and add to the cost of production. AlCl obviates these shortcomings.

This invention overcomes the disadvantages of presently used electrolytes containing A101 by providing a process wherein little, it any, AlCl is found in the electrolyte. The process comprises electrolyzing a fused salt bath composed of alkali halides between a dependent carbon or graphite anode and a cathode of molten aluminum which is denser than the electrolyte bath, simultaneously wtih the electrolysis feeding into the liquid cathode gaseous AlCl Under such circumstances, chlorine is discharged at the insoluble inert anode and aluminum is deposited and accumulated at the cathode.

The rate of A-lCl feed is synchronized with the rate of deposition of aluminum so that virtually all of the AlCl fed to the cathode is completely consumed. Although three farad ays of electricity are theoretically required to decompose the AlCl into its elements, in actual practice somewhat more than three faradays are required. The current efliciency will be above 95% reaching as high as 98.5%.

The following example will illustrate the invention:

A eutectic mixture of NaCl-KC1 melting at about 650 C. to which about by weight or Calwas added Was fused in a covered graphite lined crucible. Due to the small size of the laboratory equipment used, auxiliary external heating was provided. A high fired alumina collecting dish was placed at the bottom of the graphite liner. Aluminum metal was added piece by piece to the hath until a pool of about 1" in thickness had formed in the dish. A hollow graphite tube was lowered into the bath until it extended below the surface of the aluminum which was made cathode. The graphite liner was made anode. The cell was purged by air and vented to a chlorine absorber. With the commencement of electrolysis, gaseous AlCl was ted into the cell below the Al level 3 ,103,472 Patented Sept. 10, 1963 via the hollow graphite tube and cathode lead. The current flow through the cell was maintained at about 200 amperes. The rate of feed was erratic, but substantially no AlCl was found in the anode chlorine gas. At the end of minutes of feeding and electrolyzing, the feed was stopped and the current discontinued. The weight of total feed was about 350 grams. A metal button was separated from the frozen salts and analyzed. The results are as follows:

Temperature of electrolysis 700-750 C. Purity of AlCl 97%.

Cathode current density 16-32 ramps/ink Anode current density 2.7 amps/in Purity of aluminum 99.26% Al.

The electrolyte is composed of cations more electropositive than aluminum with respect to the halogens. Although the alkaline earth metal halides are useful, sodium and potassium halides are preferred. Similarly, although halides in general may be used, the chlorides are preterred. Small additions of alkali and/or alkaline earth fluorides seem to improve the electrolyte. The alkali chlorides are not consumed in the process but losses due to entrainment and volatilization require replenishment from time to time. The preferred anode is graphite, but carbon or other insoluble conductors may be used.

The raw material feed may be any of the halides of aluminum and as a practical matter the chloride is preterred as it is the easiest and cheapest to produce from common available ores and the easiest to handle.

Aluminum chloride is fed to the cathode preferably as a gas or vapor. All lines leading from the boiler or sublimer to the cell must be lagged to prevent condensation and plugging oi the lines. It may be desirable to pass inert gas through the lines with the AlCl gas to keep the lines open, or to stir the melts, or afford a means for controlling the feed rate in small cells. The feed rate should be controlled so that all the AlCl is consumed and so that substantialy no AlCl breaks through the surface of the aluminum cathode or distills into the chlorine phase. In the event that the rate of feed is greatly in excess of the rate of aluminum deposition, some aluminum chloride may find its way into the electrolyte. No ir-- reparable harm is done, but it is advisable to control the feed rate to prevent AlCl contaminating the chlorine gas stream or clogging the vents. The teed lines are lined internally with graphite, but it has been found that carbides, such as silicon carbides, are supen'or. The AlCl should be kept free of Water or moisture since it hydrolyzes readily leaving insoluble and highly corrosive residues.

The temperature of electrolysis should be 5010O C. above the melting point of aluminum in order to permit tapping of the aluminum product. The lower temperature of operation promotes current efficiency, lengthens cell life and reduces corrosion rates.

The current density at the liquid cathode may be varied over a wide range from a traction of an ampere per square inch to over 25 :ampercs per square inch. Similarly, the anode current density may be varied from a fraction of an ampere per square inch to over 10 amperm per square inch. However, tor the smoothest operation and tor maximum efiiciency, the anode current density should be maintained below 7 \amperes per square inch and the cathode current density below 10 amperes per square inch, 6-7 'amperes per square inch are preferred.

The reaction of the instant invention is not known precisely and the applicant does not Wish to be committed to any theory. The AlCl is not fed to the electrolyte rat The fact that little if any finds its way into the electrolyte points to its decomposition before reaching the salt bath. It may be that sodium (in an chloride electrolyte) is deposited momentarily at the interface or the electrolyte and the liquid cathode. The sodium at once reacts with the AlCl or the product of the reaction of aluminum with the A101 The reactions may be represented by:

However, from back readings, it would appear that no free sodium is deposited and that the aluminum gaseous (aluminum halide below the surface or said cathode, at .a rate to confine the aluminum halide essentially to the liquid cathode, reducing the aluminum ion to aluminum solely in said cathode to maintain the fused salt bath free of aluminum halide and passing the halide ion to said bath, thereby discharging a corresponding volatile halide gas at said insoluble anode and accumulating aluminum at the bottom of said electrolytic cell.

2. A process for the production of aluminum by fusion electrolysis comprising electrolyzing a fused salt bath composed of at least one of the group consisting of the alkali and alkaline earth chlorides and fluorides, said bath having a lesser density than molten aluminum, said electnolysis being cheated between an insoluble anode and a liquid aluminum cathode submerged in said electrolyte, simultaneously feeding gaseous aluminum chloride below the surface of said cathode at a rate to confine the aluminum chloride essentially to the liquid cathode reducing the aluminum ion to valuminum solely in said cathode to maintain the bath free of aluminum chloride and passing the chloride ion to said bath, thereby discharging chlorine at said insoluble anode and accumulating aluminum at the bottom of said electrolytic cell.

3.'A process for the production of aluminum as set forth in claim 2, wherein the rate of feed of said aluminum chloride is synchronized with the number of fara'days passing through said cell.

4. A process for the prdouction of aluminum set forth in claim 2, wherein said electrolyte comprises the alkali metal chlorides of sodium and potassium.

5. A process for the production of aluminum as set forth in claim 2, wherein said electrolysis is conducted using an anode current density below about 7 amperes per square inch and a cathode current density of about 6 amperes per square inch.

6. A process for the production of aluminum as set forth in claim 2, wherein a fluoride of the group consisting of the alkali :and alkaline earth fluorides is added to the electrolyte.

References Cited in the file of this patent UNITED STATES PATENTS 1,297,946 Weaver Mar. 18, 1919 FOREIGN PATENTS 687,758 Great Britain Feb. 18, 1953 745,530 Great Britain Feb. 29, 1956 

1. A PROCESS FOR THE PRODUCTION OF ALUMINUM BY FUSION ELECTROLYSIS COMPRISING ELECTROLYZING A FUSED SALT BATH COMPRISING AT LEAST ONE OF THE GROUP CONSISTING OF THE ALKALI AND ALKALINE EARTH HALIDES, SAID BATH HAVING A LESSER DENSITY THAN MOLTEN ALUMINUM, SAID ELECTROLYSIS BEING EFFECTED BETWEEN AN INSOLUBLE ANODE AND A LIQUID ALUMINUM CATHODE SUBMERGED IN SAID ELECTROLYTIC, SIMULTANEOUSLY FEEDING GASEOUS ALUMINUM HALIDE BELOW THE SURFACE OF SAID CATHODE, AT A RATE TO CONFINE THE ALUMINUM HALIDE ESSENTIALLY TO THE LIQUID CATHODE, REDUCING THE ALUMINUM ION TO ALYMINUM SOLELY IN SAID CATHODE TO MAINTAIN THE FUSED SALT BATH FREE OF ALUMINUM HALIDE AND PASSING THE HALIDE ION TO SAID BATH, THEREBY DISCHARGING A CORRESPONDING VOLATILE HALIDE GAS AT SAID INSOLUBLE ANODE AND ACCUMULATING ALUMINUM AT THE BOTTOM OF SAID ELECTROLYTIC CELL. 